Abstract:

A rare earth permanent magnet is prepared by disposing a powdered metal
alloy containing at least 70 vol % of an intermetallic compound phase on
a sintered body of R--Fe--B system, and heating the sintered body having
the powder disposed on its surface below the sintering temperature of the
sintered body in vacuum or in an inert gas for diffusion treatment. The
advantages include efficient productivity, excellent magnetic
performance, a minimal or zero amount of Tb or Dy used, an increased
coercive force, and a minimized decline of remanence.

Claims:

1. A method for preparing a rare earth permanent magnet, comprising the
steps of:disposing an alloy powder on a surface of a sintered body of the
composition Ra-T.sup.1.sub.b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition R.sup.1.sub.i-M.sup.1.sub.j wherein R1 is at least
one element selected from rare earth elements inclusive of Y and Sc,
M1 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb,
Hf, Ta, W, Pb, and Bi, "i" and "j" indicative of atomic percent are in
the range: 15<j≦99 and the balance of i, and containing at
least 70% by volume of an intermetallic compound phase, andheat treating
the sintered body having the powder disposed on its surface at a
temperature equal to or below the sintering temperature of the sintered
body in vacuum or in an inert gas, for causing at least one element of
R1 and M1 in the powder to diffuse to grain boundaries in the
interior of the sintered body and/or near grain boundaries within
sintered body primary phase grains.

2. The method of claim 1 wherein the disposing step includes grinding an
alloy having the composition R.sup.1.sub.i-M.sup.1.sub.j wherein R1,
M1, i and j are as defined above and containing at least 70% by
volume of an intermetallic compound phase into a powder having an average
particle size of up to 500 μm, dispersing the powder in an organic
solvent or water, applying the resulting slurry to the surface of the
sintered body, and drying.

3. The method of claim 1 wherein the heat treating step includes heat
treatment at a temperature from 200.degree. C. to (Ts-10)° C. for
1 minute to 30 hours wherein Ts represents the sintering temperature of
the sintered body.

4. The method of claim 1 wherein the sintered body has a shape including a
minimum portion with a dimension equal to or less than 20 mm.

5. A method for preparing a rare earth permanent magnet, comprising the
steps of:disposing an alloy powder on a surface of a sintered body of the
composition Ra-T.sup.1.sub.b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition R.sup.1.sub.xT.sup.2.sub.yM.sup.1.sub.z wherein R1
is at least one element selected from rare earth elements inclusive of Y
and Sc, T2 is at least one element selected from Fe and Co, M1
is at least one element selected from the group consisting of Al, Si, C,
P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta,
W, Pb, and Bi, x, y and z indicative of atomic percent are in the range:
5x≦85, 15<z≦95, and the balance of y which is greater
than 0, and containing at least 70% by volume of an intermetallic
compound phase, andheat treating the sintered body having the powder
disposed on its surface at a temperature equal to or below the sintering
temperature of the sintered body in vacuum or in an inert gas, for
causing at least one element of R1 and M1 in the powder to
diffuse to grain boundaries in the interior of the sintered body and/or
near grain boundaries within sintered body primary phase grains.

6. The method of claim 5 wherein the disposing step includes grinding an
alloy having the composition R.sup.1.sub.xT.sup.2.sub.yM.sup.1.sub.z
wherein R1, T2, M1, x, y and z are as defined above and
containing at least 70% by volume of an intermetallic compound phase into
a powder having an average particle size of up to 500 μm, dispersing
the powder in an organic solvent or water, applying the resulting slurry
to the surface of the sintered body, and drying.

7. The method of claim 5 wherein the heat treating step includes heat
treatment at a temperature from 200.degree. C. to (Ts-10)° C. for
1 minute to 30 hours wherein Ts represents the sintering temperature of
the sintered body.

8. The method of claim 5 wherein the sintered body has a shape including a
minimum portion with a dimension equal to or less than 20 mm.

9. A rare earth permanent magnet, which is prepared by disposing an alloy
powder on a surface of a sintered body of the composition
Ra-T.sup.1.sub.b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition R.sup.1.sub.i-M.sup.1.sub.j wherein R1 is at least
one element selected from rare earth elements inclusive of Y and Sc,
M1 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb,
Hf, Ta, W, Pb, and Bi, "i" and "j" indicative of atomic percent are in
the range: 15<j≦99 and the balance of i, and containing at
least 70% by volume of an intermetallic compound phase, and heat treating
the sintered body having the powder disposed on its surface at a
temperature equal to or below the sintering temperature of the sintered
body in vacuum or in an inert gas, whereinat least one element of R1
and M1 in the powder is diffused to grain boundaries in the interior
of the sintered body and/or near grain boundaries within sintered body
primary phase grains so that the coercive force of the magnet is
increased over the magnet properties of the original sintered body.

10. A rare earth permanent magnet, which is prepared by disposing an alloy
powder on a surface of a sintered body of the composition
Ra-T.sup.1.sub.b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition R.sup.1.sub.xT.sup.2.sub.yM.sup.1.sub.z wherein R1
is at least one element selected from rare earth elements inclusive of Y
and Sc, T2 is at least one element selected from Fe and Co, M1
is at least one element selected from the group consisting of Al, Si, C,
P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta,
W, Pb, and Bi, x, y and z indicative of atomic percent are in the range:
5.ltoreq.x≦85, 15<z≦95, and the balance of y which is
greater than 0, and containing at least 70% by volume of an intermetallic
compound phase, and heat treating the sintered body having the powder
disposed on its surface at a temperature equal to or below the sintering
temperature of the sintered body in vacuum or in an inert gas, whereinat
least one element of R1 and M1 in the powder is diffused to
grain boundaries in the interior of the sintered body and/or near grain
boundaries within sintered body primary phase grains so that the coercive
force of the magnet is increased over the magnet properties of the
original sintered body.

11. A method for preparing a rare earth permanent magnet, comprising the
steps of:disposing an alloy powder on a surface of a sintered body of the
composition Ra-T.sup.1.sub.b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition M.sup.1.sub.d-M.sup.2.sub.e wherein each of M1 and
M2 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In,
Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, "d" and
"e" indicative of atomic percent are in the range:
0.1.ltoreq.e≦99.9 and the balance of d, and containing at least
70% by volume of an intermetallic compound phase, andheat treating the
sintered body having the powder disposed on its surface at a temperature
equal to or below the sintering temperature of the sintered body in
vacuum or in an inert gas, for causing at least one element of M1
and M2 in the powder to diffuse to grain boundaries in the interior
of the sintered body and/or near grain boundaries within sintered body
primary phase grains.

12. The method of claim 11 wherein the disposing step includes grinding an
alloy having the composition M.sup.1.sub.d-M.sup.2.sub.e wherein M1,
M2, d and e are as defined above and containing at least 70% by
volume of an intermetallic compound phase into a powder having an average
particle size of up to 500 μm, dispersing the powder in an organic
solvent or water, applying the resulting slurry to the surface of the
sintered body, and drying.

13. The method of claim 11 wherein the heat treating step includes heat
treatment at a temperature from 200.degree. C. to (Ts-10)° C. for
1 minute to 30 hours wherein Ts represents the sintering temperature of
the sintered body.

14. The method of claim 11 wherein the sintered body has a shape including
a minimum portion with a dimension equal to or less than 20 mm.

15. A rare earth permanent magnet, which is prepared by disposing an alloy
powder on a surface of a sintered body of the composition
Ra-T.sup.1.sub.b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12.ltoreq.a≦20,
4.0.ltoreq.c≦7.0, and the balance of b, said alloy powder having
the composition M.sup.1.sub.d-M.sup.2.sub.e wherein each of M1 and
M2 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In,
Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, "d" and
"e" indicative of atomic percent are in the range:
0.1.ltoreq.e≦99.9 and the balance of d, and containing at least
70% by volume of an intermetallic compound phase, and heat treating the
sintered body having the powder disposed on its surface at a temperature
equal to or below the sintering temperature of the sintered body in
vacuum or in an inert gas, whereinat least one element of M1 and
M2 in the powder is diffused to grain boundaries in the interior of
the sintered body and/or near grain boundaries within sintered body
primary phase grains so that the coercive force of the magnet is
increased over the magnet properties of the original sintered body.

[0002]This invention relates to an R--Fe--B permanent magnet in which an
intermetallic compound is combined with a sintered magnet body so as to
enhance its coercive force while minimizing a decline of its remanence,
and a method for preparing the same.

BACKGROUND ART

[0003]By virtue of excellent magnetic properties, Nd--Fe--B permanent
magnets find an ever increasing range of application. The recent
challenge to the environmental problem has expanded the application range
of these magnets from household electric appliances to industrial
equipment, electric automobiles and wind power generators. It is required
to further improve the performance of Nd--Fe--B magnets.

[0004]Indexes for the performance of magnets include remanence (or
residual magnetic flux density) and coercive force. An increase in the
remanence of Nd--Fe--B sintered magnets can be achieved by increasing the
volume factor of Nd2Fe14B compound and improving the crystal
orientation. To this end, a number of modifications have been made. For
increasing coercive force, there are known different approaches including
grain refinement, the use of alloy compositions with greater Nd contents,
and the addition of coercivity enhancing elements such as Al and Ga. The
currently most common approach is to use alloy compositions having Dy or
Tb substituted for part of Nd.

[0005]It is believed that the coercivity creating mechanism of Nd--Fe--B
magnets is the nucleation type wherein nucleation of reverse magnetic
domains at grain boundaries governs a coercive force. In general, a
disorder of crystalline structure occurs at the grain boundary or
interface. If a disorder of crystalline structure extends several
nanometers in a depth direction near the interface of grains of
Nd2Fe14B compound which is the primary phase of the magnet,
then it incurs a lowering of magnetocrystalline anisotropy and
facilitates formation of reverse magnetic domains, reducing a coercive
force (see K. D. Durst and H. Kronmuller, "THE COERCIVE FIELD OF SINTERED
AND MELT-SPUN NdFeB MAGNETS," Journal of Magnetism and Magnetic
Materials, 68 (1987), 63-75). Substituting Dy or Tb for some Nd in the
Nd2Fe14B compound increases the anisotropic magnetic field of
the compound phase so that the coercive force is increased. When Dy or Tb
is added in an ordinary way, however, a loss of remanence is unavoidable
because Dy or Tb substitution occurs not only near the interface of the
primary phase, but even in the interior of the primary phase. Another
problem arises in that amounts of expensive Tb and Dy must be used.

[0006]Besides, a number of attempts have been made for increasing the
coercive force of Nd--Fe--B magnets. One exemplary attempt is a two-alloy
method of preparing an Nd--Fe--B magnet by mixing two powdered alloys of
different composition and sintering the mixture. A powder of alloy A
consists of R2Fe14B primary phase wherein R is mainly Nd and
Pr. And a powder of alloy B contains various additive elements including
Dy, Tb, Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb. Then alloys A and
B are mixed together. This is followed by fine pulverization, pressing in
a magnetic field, sintering, and aging treatment whereby the Nd--Fe--B
magnet is prepared. The sintered magnet thus obtained produces a high
coercive force while minimizing a decline of remanence because Dy or Tb
is absent at the center of R2Fe14B compound primary phase
grains and instead, the additive elements like Dy and Tb are localized
near grain boundaries (see JP-B 5-31807 and JP-A 5-21218). In this
method, however, Dy or Tb diffuses into the interior of primary phase
grains during the sintering so that the layer where Dy or Tb is localized
near grain boundaries has a thickness equal to or more than about 1
micrometer, which is substantially greater than the depth where
nucleation of reverse magnetic domains occurs. The results are still not
fully satisfactory.

[0007]Recently, there have been developed several processes of diffusing
certain elements from the surface to the interior of a R--Fe--B sintered
body for improving magnet properties. In one exemplary process, a rare
earth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on the
surface of Nd--Fe--B magnet using an evaporation or sputtering technique,
followed by heat treatment. See JP-A 2004-296973, JP-A 2004-304038, JP-A
2005-11973; K. T. Park, K. Hiraga and M. Sagawa, "Effect of Metal-Coating
and Consecutive Heat Treatment on Coercivity of Thin Nd--Fe--B Sintered
Magnets," Proceedings of the 16th International Workshop on Rare-Earth
Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida, et
al., "Grain Boundary Modification of Nd--Fe--B Sintered Magnet and
Magnetic Properties," Abstracts of Spring Meeting of Japan Society of
Powder and Powder Metallurgy, 2004, p. 202. Another exemplary process
involves applying a powder of rare earth inorganic compound such as
fluoride or oxide onto the surface of a sintered body and heat treatment
as described in WO 2006/043348 A1. With these processes, the element
(e.g., Dy or Tb) disposed on the sintered body surface pass through grain
boundaries in the sintered body structure and diffuse into the interior
of the sintered body during the heat treatment. As a consequence, Dy or
Tb can be enriched in a very high concentration at grain boundaries or
near grain boundaries within sintered body primary phase grains. As
compared with the two-alloy method described previously, these processes
produce an ideal morphology. Since the magnet properties reflect the
morphology, a minimized decline of remanence and an increase of coercive
force are accomplished. However, the processes utilizing evaporation or
sputtering have many problems associated with units and steps when
practiced on a mass scale and suffer from poor productivity.

DISCLOSURE OF THE INVENTION

[0008]An object of the invention is to provide an R--Fe--B sintered magnet
which is prepared by applying an intermetallic compound-based alloy
powder onto a sintered body and effecting diffusion treatment and which
magnet features efficient productivity, excellent magnetic performance, a
minimal or zero amount of Tb or Dy used, an increased coercive force, and
a minimized decline of remanence. Another object is to provide a method
for preparing the same.

[0009]The inventors have discovered that when an R--Fe--B sintered body is
tailored by applying to a surface thereof an alloy powder based on an
easily pulverizable intermetallic compound phase and effecting diffusion
treatment, the process is improved in productivity over the prior art
processes, and constituent elements of the diffusion alloy are enriched
near the interface of primary phase grains within the sintered body so
that the coercive force is increased while minimizing a decline of
remanence. The invention is predicated on this discovery.

[0011]disposing an alloy powder on a surface of a sintered body of the
composition Ra-T1b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition R1i-M1j wherein R1 is at least
one element selected from rare earth elements inclusive of Y and Sc,
M1 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb,
Hf, Ta, W, Pb, and Bi, "i" and "j" indicative of atomic percent are in
the range: 15<j≦99 and the balance of i, and containing at
least 70% by volume of an intermetallic compound phase, and

[0012]heat treating the sintered body having the powder disposed on its
surface at a temperature equal to or below the sintering temperature of
the sintered body in vacuum or in an inert gas, for causing at least one
element of R1 and M1 in the powder to diffuse to grain
boundaries in the interior of the sintered body and/or near grain
boundaries within sintered body primary phase grains.

[2] The method of [1] wherein the disposing step includes grinding an
alloy having the composition R1i-M1j wherein R1,
M1, i and j are as defined above and containing at least 70% by
volume of an intermetallic compound phase into a powder having an average
particle size of up to 500 μm, dispersing the powder in an organic
solvent or water, applying the resulting slurry to the surface of the
sintered body, and drying.[3] The method of [1] or [2] wherein the heat
treating step includes heat treatment at a temperature from 200°
C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts represents
the sintering temperature of the sintered body.[4] The method of [1], [2]
or [3] wherein the sintered body has a shape including a minimum portion
with a dimension equal to or less than 20 mm.[5] A method for preparing a
rare earth permanent magnet, comprising the steps of:

[0013]disposing an alloy powder on a surface of a sintered body of the
composition Ra-T1b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition R1xT2yM1z wherein R1
is at least one element selected from rare earth elements inclusive of Y
and Sc, T2 is at least one element selected from Fe and Co, M1
is at least one element selected from the group consisting of Al, Si, C,
P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta,
W, Pb, and Bi, x, y and z indicative of atomic percent are in the range:
5≦x≦85, 15<z≦95, and the balance of y which is
greater than 0, and containing at least 70% by volume of an intermetallic
compound phase, and

[0014]heat treating the sintered body having the powder disposed on its
surface at a temperature equal to or below the sintering temperature of
the sintered body in vacuum or in an inert gas, for causing at least one
element of R1 and M1 in the powder to diffuse to grain
boundaries in the interior of the sintered body and/or near grain
boundaries within sintered body primary phase grains.

[6] The method of [5] wherein the disposing step includes grinding an
alloy having the composition R1xT2yM1z
wherein R1, T2, M1, x, y and z are as defined above and
containing at least 70% by volume of an intermetallic compound phase into
a powder having an average particle size of up to 500 μm, dispersing
the powder in an organic solvent or water, applying the resulting slurry
to the surface of the sintered body, and drying.[7] The method of [5] or
[6] wherein the heat treating step includes heat treatment at a
temperature from 200° C. to (Ts-10)° C. for 1 minute to 30
hours wherein Ts represents the sintering temperature of the sintered
body.[8] The method of [5], [6] or [7] wherein the sintered body has a
shape including a minimum portion with a dimension equal to or less than
20 mm.[9] A rare earth permanent magnet, which is prepared by disposing
an alloy powder on a surface of a sintered body of the composition
Ra-T1b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition R1i-M1j wherein R1 is at least
one element selected from rare earth elements inclusive of Y and Sc,
M1 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb,
Hf, Ta, W, Pb, and Bi, "i" and "j" indicative of atomic percent are in
the range: 15<j≦99 and the balance of i, and containing at
least 70% by volume of an intermetallic compound phase, and heat treating
the sintered body having the powder disposed on its surface at a
temperature equal to or below the sintering temperature of the sintered
body in vacuum or in an inert gas, wherein

[0015]at least one element of R1 and M1 in the powder is
diffused to grain boundaries in the interior of the sintered body and/or
near grain boundaries within sintered body primary phase grains so that
the coercive force of the magnet is increased over the magnet properties
of the original sintered body.

[10] A rare earth permanent magnet, which is prepared by disposing an
alloy powder on a surface of a sintered body of the composition
Ra-T1b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition R1xT2yM1z wherein R1
is at least one element selected from rare earth elements inclusive of Y
and Sc, T2 is at least one element selected from Fe and Co, M1
is at least one element selected from the group consisting of Al, Si, C,
P, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta,
W, Pb, and Bi, x, y and z indicative of atomic percent are in the range:
5≦x≦85, 15<z≦95, and the balance of y which is
greater than 0, and containing at least 70% by volume of an intermetallic
compound phase, and heat treating the sintered body having the powder
disposed on its surface at a temperature equal to or below the sintering
temperature of the sintered body in vacuum or in an inert gas, wherein

[0016]at least one element of R1 and M1 in the powder is
diffused to grain boundaries in the interior of the sintered body and/or
near grain boundaries within sintered body primary phase grains so that
the coercive force of the magnet is increased over the magnet properties
of the original sintered body.

[0017]disposing an alloy powder on a surface of a sintered body of the
composition Ra-T1b-Bc wherein R is at least one
element selected from rare earth elements inclusive of Y and Sc, T1
is at least one element selected from Fe and Co, B is boron, "a," "b" and
"c" indicative of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition M1d-M2e wherein each of M1 and
M2 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In,
Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, "d" and
"e" indicative of atomic percent are in the range:
0.1≦e≦99.9 and the balance of d, and containing at least
70% by volume of an intermetallic compound phase, and

[0018]heat treating the sintered body having the powder disposed on its
surface at a temperature equal to or below the sintering temperature of
the sintered body in vacuum or in an inert gas, for causing at least one
element of M1 and M2 in the powder to diffuse to grain
boundaries in the interior of the sintered body and/or near grain
boundaries within sintered body primary phase grains.

[12] The method of [11] wherein the disposing step includes grinding an
alloy having the composition M1d-M2e wherein M1,
M2, d and e are as defined above and containing at least 70% by
volume of an intermetallic compound phase into a powder having an average
particle size of up to 500 μm, dispersing the powder in an organic
solvent or water, applying the resulting slurry to the surface of the
sintered body, and drying.[13] The method of [11] or [12] wherein the
heat treating step includes heat treatment at a temperature from
200° C. to (Ts-10)° C. for 1 minute to 30 hours wherein Ts
represents the sintering temperature of the sintered body.[14] The method
of [11], [12] or [13] wherein the sintered body has a shape including a
minimum portion with a dimension equal to or less than 20 mm.[15] A rare
earth permanent magnet, which is prepared by disposing an alloy powder on
a surface of a sintered body of the composition
Ra-T1b-Bc wherein R is at least one element selected
from rare earth elements inclusive of Y and Sc, T1 is at least one
element selected from Fe and Co, B is boron, "a," "b" and "c" indicative
of atomic percent are in the range: 12≦a≦20,
4.0≦c≦7.0, and the balance of b, said alloy powder having
the composition M1d-M2e wherein each of M1 and
M2 is at least one element selected from the group consisting of Al,
Si, C, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In,
Sn, Sb, Hf, Ta, W, Pb, and Bi, M1 is different from M2, "d" and
"e" indicative of atomic percent are in the range:
0.1≦e≦99.9 and the balance of d, and containing at least
70% by volume of an intermetallic compound phase, and heat treating the
sintered body having the powder disposed on its surface at a temperature
equal to or below the sintering temperature of the sintered body in
vacuum or in an inert gas, wherein

[0019]at least one element of M1 and M2 in the powder is
diffused to grain boundaries in the interior of the sintered body and/or
near grain boundaries within sintered body primary phase grains so that
the coercive force of the magnet is increased over the magnet properties
of the original sintered body.

BENEFITS OF THE INVENTION

[0020]According to the invention, an R--Fe--B sintered magnet is prepared
by applying an alloy powder based on an easily pulverizable intermetallic
compound onto a sintered body and effecting diffusion treatment. The
advantages of the resultant magnet include efficient productivity,
excellent magnetic performance, a minimal or zero amount of Tb or Dy
used, an increased coercive force, and a minimized decline of remanence.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021]Briefly stated, an R--Fe--B sintered magnet is prepared according to
the invention by applying an intermetallic compound-based alloy powder
onto a sintered body and effecting diffusion treatment. The resultant
magnet has advantages including excellent magnetic performance and a
minimal amount of Tb or Dy used or the absence of Tb or Dy.

[0022]The mother material used in the invention is a sintered body of the
composition Ra-T1b-Bc, which is often referred to as
"mother sintered body." Herein R is at least one element selected from
rare earth elements inclusive of scandium (Sc) and yttrium (Y),
specifically from among Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Yb, and Lu. Preferably the majority of R is Nd and/or Pr. Preferably
the rare earth elements inclusive of Sc and Y account for 12 to 20 atomic
percents (at %), and more preferably 14 to 18 at % of the entire sintered
body. T1 is at least one element selected from iron (Fe) and cobalt
(Co). B is boron, and preferably accounts for 4 to 7 at % of the entire
sintered body. Particularly when B is 5 to 6 at %, a significant
improvement in coercive force is achieved by diffusion treatment. The
balance consists of T1.

[0023]The alloy for the mother sintered body is prepared by melting metal
or alloy feeds in vacuum or an inert gas atmosphere, preferably argon
atmosphere, and casting the melt into a flat mold or book mold or strip
casting. A possible alternative is a so-called two-alloy process
involving separately preparing an alloy approximate to the
R2Fe14B compound composition constituting the primary phase of
the relevant alloy and a rare earth-rich alloy serving as a liquid phase
aid at the sintering temperature, crushing, then weighing and mixing
them. Notably, the alloy approximate to the primary phase composition is
subjected to homogenizing treatment, if necessary, for the purpose of
increasing the amount of the R2Fe14B compound phase, since
primary crystal α-Fe is likely to be left depending on the cooling
rate during casting and the alloy composition. The homogenizing treatment
is a heat treatment at 700 to 1,200° C. for at least one hour in
vacuum or in an Ar atmosphere. Alternatively, the alloy approximate to
the primary phase composition may be prepared by the strip casting
technique. To the rare earth-rich alloy serving as a liquid phase aid,
the melt quenching and strip casting techniques are applicable as well as
the above-described casting technique.

[0024]The alloy is generally crushed or coarsely ground to a size of 0.05
to 3 mm, especially 0.05 to 1.5 mm. The crushing step uses a Brown mill
or hydriding pulverization, with the hydriding pulverization being
preferred for those alloys as strip cast. The coarse powder is then
finely pulverized to an average particle size of 0.2 to 30 μm,
especially 0.5 to 20 μm, for example, on a jet mill using
high-pressure nitrogen.

[0025]The fine powder is compacted on a compression molding machine under
a magnetic field. The green compact is then placed in a sintering furnace
where it is sintered in vacuum or in an inert gas atmosphere usually at a
temperature of 900 to 1,250° C., preferably 1,000 to 1,100°
C. The sintered block thus obtained contains 60 to 99% by volume,
preferably 80 to 98% by volume of the tetragonal R2Fe14B
compound as the primary phase, with the balance being 0.5 to 20% by
volume of a rare earth-rich phase and 0.1 to 10% by volume of at least
one compound selected from among rare earth oxides, and carbides,
nitrides and hydroxides of incidental impurities, and mixtures or
composites thereof.

[0026]The resulting sintered block may be machined or worked into a
predetermined shape. In the invention, R1 and/or M1 and
T2, or M1 and/or M2 which are to be diffused into the
sintered body interior are supplied from the sintered body surface. Thus,
if a minimum portion of the sintered body has too large a dimension, the
objects of the invention are not achievable. For this reason, the shape
includes a minimum portion having a dimension equal to or less than 20
mm, and preferably equal to or less than 10 mm, with the lower limit
being equal to or more than 0.1 mm. The sintered body includes a maximum
portion whose dimension is not particularly limited, with the maximum
portion dimension being desirably equal to or less than 200 mm.

[0027]According to the invention, an alloy powder is disposed on the
sintered body and subjected to diffusion treatment. It is a powdered
alloy having the composition:

[0028]R11-M1j or
R1xT2yM1z or M1d-M2e.
This alloy is often referred to as "diffusion alloy." Herein R1 is
at least one element selected from rare earth elements inclusive of Y and
Sc, and preferably the majority of R1 is Nd and Pr. M1 is at
least one element selected from the group consisting of Al, Si, C, P, Ti,
V, Cr, Mn, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Pb,
and Bi. In the alloy M1d-M2e, M1 and M2 are
different from each other and selected from the group consisting of the
foregoing elements. T2 is Fe and/or Co. In the alloy
R1i-M1j, M1 accounts for 15 to 99 at % (i.e.,
j=15 to 99), with the balance being R1. In the alloy
R1xT2yM1z, M1 accounts for 15 to 95 at
% (i.e., z=15 to 95) and R1 accounts for 5 to 85 at % (i.e., x=5 to
85), with the balance being T2. That is, y>0, and T2 is
preferably 0.5 to 75 at %. In the alloy M1d-M2e,
M2 accounts for 0.1 to 99.9 at %, that is, e is in the range:
0.1≦e≦99.9. M1 is the remainder after removal of
M2, that is, d is the balance.

[0029]The diffusion alloy may contain incidental impurities such as
nitrogen (N) and oxygen (O), with an acceptable total amount of such
impurities being equal to or less than 4 at %.

[0030]The invention is characterized in that the diffusion alloy material
contains at least 70% by volume of an intermetallic compound phase in its
structure. If the diffusion material is composed of a single metal or
eutectic alloy, it is unsusceptible to pulverization and requires a
special technique such as atomizing for a fine powder. By contrast, the
intermetallic compound phase is generally hard and brittle in nature.
When an alloy based on such an intermetallic compound phase is used as
the diffusion material, a fine powder is readily obtained simply by
applying the alloy preparation or pulverization means used in the
manufacture of R--Fe--B sintered magnets. This is quite advantageous from
the productivity aspect. Since the diffusion alloy material is
advantageously readily pulverizable, it preferably contains at least 70%
by volume and more preferably at least 90% by volume of an intermetallic
compound phase. It is understood that the term "% by volume" is
interchangeable with a percent by area of an intermetallic compound phase
in a cross-section of the alloy structure.

[0031]The diffusion alloy containing at least 70% by volume of the
intermetallic compound phase represented by R1i-M1j,
R1xT2yM1z or M1d-M2e
may be prepared, like the alloy for the mother sintered body, by melting
metal or alloy feeds in vacuum or an inert gas atmosphere, preferably
argon atmosphere, and casting the melt into a flat mold or book mold. An
arc melting or strip casting method is also acceptable. The alloy is then
crushed or coarsely ground to a size of about 0.05 to 3 mm, especially
about 0.05 to 1.5 mm by means of a Brown mill or hydriding pulverization.
The coarse powder is then finely pulverized, for example, by a ball mill,
vibration mill or jet mill using high-pressure nitrogen. The smaller the
powder particle size, the higher becomes the diffusion efficiency. The
diffusion alloy containing the intermetallic compound phase represented
by R1i-M1j, R1xT2yM1z
or M1d-M2e, when powdered, preferably has an average
particle size equal to or less than 500 μm, more preferably equal to
or less than 300 μm, and even more preferably equal to or less than
100 μm. However, if the particle size is too small, then the influence
of surface oxidation becomes noticeable, and handling is dangerous. Thus
the lower limit of average particle size is preferably equal to or more
than 1 μm. As used herein, the "average particle size" may be
determined as a weight average diameter D50 (particle diameter at
50% by weight cumulative, or median diameter) using, for example, a
particle size distribution measuring instrument relying on laser
diffractometry or the like.

[0032]After the powder of diffusion alloy is disposed on the surface of
the mother sintered body, the mother sintered body and the diffusion
alloy powder are heat treated in vacuum or in an atmosphere of an inert
gas such as argon (Ar) or helium (He) at a temperature equal to or below
the sintering temperature (designated Ts in ° C.) of the sintered
body. This heat treatment is referred to as "diffusion treatment." By the
diffusion treatment, R1, M1 or M2 in the diffusion alloy
is diffused to grain boundaries in the interior of the sintered body
and/or near grain boundaries within sintered body primary phase grains.

[0033]The diffusion alloy powder is disposed on the surface of the mother
sintered body, for example, by dispersing the powder in water or an
organic solvent to form a slurry, immersing the sintered body in the
slurry, and drying the immersed sintered body by air drying, hot air
drying or in vacuum. Spray coating is also possible. The slurry may
contain 1 to 90% by weight, and preferably 5 to 70% by weight of the
powder.

[0034]Better results are obtained when the filling factor of the elements
from the applied diffusion alloy is at least 1% by volume, preferably at
least 10% by volume, calculated as an average value in a sintered
body-surrounding space extending outward from the sintered body surface
to a distance equal to or less than 1 mm. The upper limit of filling
factor is generally equal to or less than 95% by volume, and preferably
equal to or less than 90% by volume, though not critical.

[0035]The conditions of diffusion treatment vary with the type and
composition of the diffusion alloy and are preferably selected such that
R1 and/or M1 and/or M2 is enriched at grain boundaries in
the interior of the sintered body and/or near grain boundaries within
sintered body primary phase grains. The temperature of diffusion
treatment is equal to or below the sintering temperature (designated Ts
in ° C.) of the sintered body. If diffusion treatment is effected
above Ts, there arise problems that (1) the structure of the sintered
body can be altered to degrade magnetic properties, and (2) the machined
dimensions cannot be maintained due to thermal deformation. For this
reason, the temperature of diffusion treatment is equal to or below
Ts° C. of the sintered body, and preferably equal to or below
(Ts-10)° C. The lower limit of temperature may be selected as
appropriate though it is typically at least 200° C., and
preferably at least 350° C. The time of diffusion treatment is
typically from 1 minute to 30 hours. Within less than 1 minute, the
diffusion treatment is not complete. If the treatment time is over 30
hours, the structure of the sintered body can be altered, oxidation or
evaporation of components inevitably occurs to degrade magnetic
properties, or M1 or M2 is not only enriched at grain
boundaries in the interior of the sintered body and/or near grain
boundaries within sintered body primary phase grains, but also diffused
into the interior of primary phase grains. The preferred time of
diffusion treatment is from 1 minute to 10 hours, and more preferably
from 10 minutes to 6 hours.

[0036]Through appropriate diffusion treatment, the constituent element
R1, M1 or M2 of the diffusion alloy disposed on the
surface of the sintered body is diffused into the sintered body while
traveling mainly along grain boundaries in the sintered body structure.
This results in the structure in which R1, M1 or M2 is
enriched at grain boundaries in the interior of the sintered body and/or
near grain boundaries within sintered body primary phase grains.

[0037]The permanent magnet thus obtained is improved in coercivity in that
the diffusion of R1, M1 or M2 modifies the morphology near
the primary phase grain boundaries within the structure so as to suppress
a decline of magnetocrystalline anisotropy at primary phase grain
boundaries or to create a new phase at grain boundaries. Since the
diffusion alloy elements have not diffused into the interior of primary
phase grains, a decline of remanence is restrained. The magnet is a high
performance permanent magnet.

[0038]After the diffusion treatment, the magnet may be further subjected
to aging treatment at a temperature of 200 to 900° C. for
augmenting the coercivity enhancement.

EXAMPLE

[0039]Examples are given below for further illustrating the invention
although the invention is not limited thereto.

Example 1 and Comparative Example 1

[0040]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0041]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 5.2 μm. The fine powder was compacted under a
pressure of about 300 kg/cm2 while being oriented in a magnetic
field of 1592 kAm-1. The green compact was then placed in a vacuum
sintering furnace where it was sintered at 1,060° C. for 1.5
hours, obtaining a sintered block. Using a diamond grinding tool, the
sintered block was machined on all the surfaces into a shape having
dimensions of 4×4×2 mm. It was washed in sequence with
alkaline solution, deionized water, nitric acid and deionized water, and
dried, obtaining a mother sintered body which had the composition
Nd16.0FebalCo1.0B5.3.

[0042]By using Nd and Al metals having a purity of at least 99% by weight
and arc melting in an argon atmosphere, a diffusion alloy having the
composition Nd33Al67 and composed mainly of an intermetallic
compound phase NdAl2 was prepared. The alloy was finely pulverized
on a ball mill using an organic solvent into a fine powder having a mass
median particle diameter of 7.8 μm. On electron probe microanalysis
(EPMA), the alloy contained 94% by volume of the intermetallic compound
phase NdAl2.

[0043]The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to
form a slurry, in which the mother sintered body was immersed for 30
seconds under ultrasonic agitation. The sintered body was pulled up and
immediately dried with hot air.

[0044]The sintered body covered with the diffusion alloy powder was
subjected to diffusion treatment in vacuum at 800° C. for one
hour, yielding a magnet of Example 1. In the absence of the diffusion
alloy powder, the sintered body alone was subjected to heat treatment in
vacuum at 800° C. for one hour, yielding a magnet of Comparative
Example 1.

[0045]Table 1 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compound in the diffusion
alloy, the temperature and time of diffusion treatment in Example 1 and
Comparative Example 1. Table 2 shows the magnetic properties of the
magnets of Example 1 and Comparative Example 1. It is seen that the
coercive force (Hcj) of the magnet of Example 1 is greater by 1300
kAm-1 than that of Comparative Example 1 while a decline of
remanence (Br) is only 15 mT.

[0046]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0047]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 5.2 μm. The fine powder was compacted under a
pressure of about 300 kg/cm2 while being oriented in a magnetic
field of 1592 kAm-1. The green compact was then placed in a vacuum
sintering furnace where it was sintered at 1,060° C. for 1.5
hours, obtaining a sintered block. Using a diamond grinding tool, the
sintered block was machined on all the surfaces into a shape having
dimensions of 4×4×2 mm. It was washed in sequence with
alkaline solution, deionized water, nitric acid and deionized water, and
dried, obtaining a mother sintered body which had the composition
Nd16.0FebalCo1.0B5.3.

[0048]By using Nd, Fe, Co and Al metals having a purity of at least 99% by
weight and arc melting in an argon atmosphere, a diffusion alloy having
the composition Nd35Fe21Co20Al20 was prepared. The
alloy was finely pulverized on a ball mill using an organic solvent into
a fine powder having a mass median particle diameter of 7.8 μm. On
EPMA analysis, the alloy contained intermetallic compound phases
Nd(FeCoAl)2, Nd2(FeCoAl) and Nd2(FeCoAl)17 and the
like, with the total of intermetallic compound phases being 87% by
volume.

[0049]The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to
form a slurry, in which the mother sintered body was immersed for 30
seconds under ultrasonic agitation. The sintered body was pulled up and
immediately dried with hot air.

[0050]The sintered body covered with the diffusion alloy powder was
subjected to diffusion treatment in vacuum at 800° C. for one
hour, yielding a magnet of Example 2. In the absence of the powdered
diffusion alloy, the sintered body alone was subjected to heat treatment
in vacuum at 800° C. for one hour, yielding a magnet of
Comparative Example 2.

[0051]Table 3 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compounds in the diffusion
alloy, the temperature and time of diffusion treatment in Example 2 and
Comparative Example 2. Table 4 shows the magnetic properties of the
magnets of Example 2 and Comparative Example 2. It is seen that the
coercive force of the magnet of Example 2 is greater by 1150 kAm-1
than that of Comparative Example 2 while a decline of remanence is only
18 mT.

[0052]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0053]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 5.2 μm. The fine powder was compacted under a
pressure of about 300 kg/cm2 while being oriented in a magnetic
field of 1592 kAm-1. The green compact was then placed in a vacuum
sintering furnace where it was sintered at 1,060° C. for 1.5
hours, obtaining a sintered block. Using a diamond grinding tool, the
sintered block was machined on all the surfaces into a shape having
dimensions of 50×50×15 mm (Example 3-1) or a shape having
dimensions of 50×50×25 mm (Example 3-2). It was washed in
sequence with alkaline solution, deionized water, nitric acid and
deionized water, and dried, obtaining a mother sintered body which had
the composition Nd16.0FebalCo1.0B5.3.

[0054]By using Nd and Al metals having a purity of at least 99% by weight
and arc melting in an argon atmosphere, a diffusion alloy having the
composition Nd33Al67 and composed mainly of an intermetallic
compound phase NdAl2 was prepared. The alloy was finely pulverized
on a ball mill using an organic solvent into a fine powder having a mass
median particle diameter of 7.8 μm. On EPMA analysis, the alloy
contained 93% by volume of the intermetallic compound phase NdAl2.

[0055]The diffusion alloy powder, 30 g, was mixed with 90 g of ethanol to
form a slurry, in which each mother sintered body of Examples 3-1 and 3-2
was immersed for 30 seconds under ultrasonic agitation. The sintered body
was pulled up and immediately dried with hot air.

[0056]The sintered bodies covered with the diffusion alloy powder were
subjected to diffusion treatment in vacuum at 850° C. for 6 hours,
yielding magnets of Example 3-1 and 3-2.

[0057]Table 5 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compound in the diffusion
alloy, the temperature and time of diffusion treatment, and the dimension
of sintered body minimum portion in Examples 3-1 and 3-2. Table 6 shows
the magnetic properties of the magnets of Examples 3-1 and 3-2. It is
seen that in Example 3-1 where the sintered body minimum portion had a
dimension of 15 mm, the diffusion treatment exerted a greater effect as
demonstrated by a coercive force of 1584 kAm-1. In contrast, where
the sintered body minimum portion had a dimension in excess of 20 mm, for
example, a dimension of 25 mm in Example 3-2, the diffusion treatment
exerted a less effect.

[0058]As in Example 1, various mother sintered bodies were coated with
various diffusion alloys and subjected to diffusion treatment at certain
temperatures for certain times. Tables 7 and 8 summarize the composition
of the mother sintered body and the diffusion alloy, the type and amount
of main intermetallic compound in the diffusion alloy, the temperature
and time of diffusion treatment. Tables 9 and 10 show the magnetic
properties of the magnets. It is noted that the amount of intermetallic
compound in the diffusion alloy was determined by EPMA analysis.

[0059]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0060]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 5.2 μm. The fine powder was compacted under a
pressure of about 300 kg/cm2 while being oriented in a magnetic
field of 1592 kAm-1. The green compact was then placed in a vacuum
sintering furnace where it was sintered at 1,060° C. for 1.5
hours, obtaining a sintered block. Using a diamond grinding tool, the
sintered block was machined on all the surfaces into a shape having
dimensions of 4×4×2 mm. It was washed in sequence with
alkaline solution, deionized water, nitric acid and deionized water, and
dried, obtaining a mother sintered body which had the composition
Nd16.0FebalCo1.0B5.3.

[0061]By using Al and Co metals having a purity of at least 99% by weight
and arc melting in an argon atmosphere, a diffusion alloy having the
composition Al50Co50 (in atom %) and composed mainly of an
intermetallic compound phase AlCo was prepared. The alloy was finely
pulverized on a ball mill using an organic solvent into a fine powder
having a mass median particle diameter of 8.5 μm. On EPMA analysis,
the alloy contained 93% by volume of the intermetallic compound phase
AlCo.

[0062]The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to
form a slurry, in which the mother sintered body was immersed for 30
seconds under ultrasonic agitation. The sintered body was pulled up and
immediately dried with hot air.

[0063]The sintered body covered with the diffusion alloy powder was
subjected to diffusion treatment in vacuum at 800° C. for one
hour, yielding a magnet of Example 53.

[0064]Table 11 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compound in the diffusion
alloy, the temperature and time of diffusion treatment in Example 53.
Table 12 shows the magnetic properties of the magnet of Example 53. It is
seen that the coercive force of the magnet of Example 53 is greater by
1170 kAm-1 than that of the preceding Comparative Example 1 while a
decline of remanence is only 20 mT.

[0065]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0066]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 5.2 μm. The fine powder was compacted under a
pressure of about 300 kg/cm2 while being oriented in a magnetic
field of 1592 kAm-1. The green compact was then placed in a vacuum
sintering furnace where it was sintered at 1,060° C. for 1.5
hours, obtaining a sintered block. Using a diamond grinding tool, the
sintered block was machined on all the surfaces into a shape having
dimensions of 50×50×15 mm (Example 54) or a shape having
dimensions of 50×50×25 mm (Comparative Example 3). It was
washed in sequence with alkaline solution, deionized water, nitric acid
and deionized water, and dried, obtaining a mother sintered body which
had the composition Nd16.0FebalCo1.0B5.3.

[0067]By using Al and Co metals having a purity of at least 99% by weight
and arc melting in an argon atmosphere, a diffusion alloy having the
composition Al50Co50 (in atom %) and composed mainly of an
intermetallic compound phase AlCo was prepared. The alloy was finely
pulverized on a ball mill using an organic solvent into a fine powder
having a mass median particle diameter of 8.5 μm. On EPMA analysis,
the alloy contained 92% by volume of the intermetallic compound phase
AlCo.

[0068]The diffusion alloy powder, 30 g, was mixed with 90 g of ethanol to
form a slurry, in which each mother sintered body of Example 54 and
Comparative Example 3 was immersed for 30 seconds under ultrasonic
agitation. The sintered body was pulled up and immediately dried with hot
air.

[0070]Table 13 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compound in the diffusion
alloy, the temperature and time of diffusion treatment, and the dimension
of sintered body minimum portion in Example 54 and Comparative Example 3.
Table 14 shows the magnetic properties of the magnets of Example 54 and
Comparative Example 3. It is seen that in Example 54 where the sintered
body minimum portion had a dimension of 15 mm, the diffusion treatment
exerted a greater effect as demonstrated by a coercive force of 1504
kAm-1. In contrast, where the sintered body minimum portion had a
dimension in excess of 20 mm, for example, a dimension of 25 mm in
Comparative Example 3, the diffusion treatment exerted little effect as
demonstrated by little increase of coercive force.

[0071]As in Example 53, various mother sintered bodies were coated with
various diffusion alloy powder and subjected to diffusion treatment at
certain temperatures for certain times. Table 15 summarizes the
composition of the mother sintered body and the diffusion alloy, the type
and amount of main intermetallic compound phase in the diffusion alloy,
the temperature and time of diffusion treatment. Table 16 shows the
magnetic properties of the magnets. It is noted that the amount of
intermetallic compound phase in the diffusion alloy was determined by
EPMA analysis.

[0072]A magnet alloy was prepared by using Nd, Fe and Co metals having a
purity of at least 99% by weight and ferroboron, high-frequency heating
in an argon atmosphere for melting, and casting the alloy melt in a
copper mold. The alloy was ground on a Brown mill into a coarse powder
with a particle size of up to 1 mm.

[0073]Subsequently, the coarse powder was finely pulverized on a jet mill
using high-pressure nitrogen gas into a fine powder having a mass median
particle diameter of 4.2 μm. The atmosphere was changes to an inert
gas so that the oxidation of the fine powder is inhibited. Then, the fine
powder was compacted under a pressure of about 300 kg/cm2 while
being oriented in a magnetic field of 1592 kAm-1. The green compact
was then placed in a vacuum sintering furnace where it was sintered at
1,060° C. for 1.5 hours, obtaining a sintered block. Using a
diamond grinding tool, the sintered block was machined on all the
surfaces into a shape having dimensions of 4×4×2 mm. It was
washed in sequence with alkaline solution, deionized water, nitric acid
and deionized water, and dried, obtaining a mother sintered body which
had the composition Nd13.8FebalCo1.0B6.0.

[0074]By using Dy, Tb, Nd, Pr, Co, Ni and Al metals having a purity of at
least 99% by weight and arc melting in an argon atmosphere, diffusion
alloys having various compositions (in atom %) as shown in Table 17 were
prepared. Each alloy was finely pulverized on a ball mill using an
organic solvent into a fine powder having a mass median particle diameter
of 7.9 μm. On EPMA analysis, each alloy contained 94% by volume of the
intermetallic compound phase shown in Table 17.

[0075]The diffusion alloy powder, 15 g, was mixed with 45 g of ethanol to
form a slurry, in which each mother sintered body was immersed for 30
seconds under ultrasonic agitation. The sintered body was pulled up and
immediately dried with hot air.

[0076]The sintered bodies covered with the diffusion alloy powder were
subjected to diffusion treatment in vacuum at 840° C. for 10
hours, yielding magnets of Examples 85 to 92. A magnet of Comparative
Example 4 was also obtained by repeating the above procedure except the
diffusion alloy powder was not used.

[0077]Table 17 summarizes the composition of the mother sintered body and
the diffusion alloy, the main intermetallic compound in the diffusion
alloy, and the temperature and time of diffusion treatment in Examples 85
to 92 and Comparative Example 4. Table 18 shows the magnetic properties
of the magnets of Examples 85 to 92 and Comparative Example 4. It is seen
that the coercive force of the magnets of Examples 85 to 92 is
considerably greater than that of Comparative Example 4, while a decline
of remanence is only about 10 mT.

[0079]Although some preferred embodiments have been described, many
modifications and variations may be made thereto in light of the above
teachings. It is therefore to be understood that the invention may be
practiced otherwise than as specifically described without departing from
the scope of the appended claims.